Threads or yarns produced from spindles in ring or rotor spinning frames are "cleaned" i.e., controlled for thickness, before being wound on spools. Thick and thin parts outside tolerances--i.e. unacceptably defective--are excised and the sections on either side of the removed flaw are rejoined by knotting or splicing. Those flaws which deviate greatly from the nominal value, i.e., from the rated yarn thickness, are excised even when they are comparatively short, whereas defects deviating comparatively slightly from the nominal value are excised only if they are comparatively long. Accordingly, cleansing is accomplished when a yarn defect exceeds compound values formed on one hand by deviation from the nominal yarn thickness and on the other hand by the length of the deviation.
Moreover, such yarn defects can be "graded". This means that a two-dimensional reference is associated with each thick or thin length--that is, with each acceptable and unacceptable defect--which reflects both the deviation from the nominal value and how long the deviation is. As a result, a curve can be developed such that, in a two-dimensional plane formed from the two-dimensional reference called the "grading plane", there will be an arbitrary curve determining the elimination from the yarn of all thick and thin lengths outside the cleansing "envelopes" and hence which are considered unacceptable defects.
Depending on how the two-dimensional numerical reference and the shape of the cleansing curve are constituted (the two magnitudes can be selected in different ways), it is possible to manufacture yarn with the same thickness distribution and with significantly more or fewer knots or splices. Accordingly, the fewer knots or splicings which have been selected per unit length of yarn, and the closer together the cleansing boundaries of the unacceptable defects, the higher the yarn quality.
Swiss patent 477,573 describes apparatus for a yarn cleanser wherein the yarn thickness is converted into an electrical signal which then is fed to a damping circuit with an adjustable time-constant and then to a Schmitt trigger of which the response threshold also is adjustable. Signals thus processed which exceed a specified pulse height, representing the magnitude of the yarn defect, actuate a cutting and knot-tieing mechanism. Accordingly, only those yarn defects are detected which are considered unacceptable by the apparatus. Depending on the time-constant, short thick segments and long slowly growing thick segments are, respectively, substantially undamped and substantially damped before being fed to the Schmitt trigger, as a result of which short segments are excised only if there is substantial deviation from the nominal value, while long, gradually growing segments on the other hand are excised even if they deviate only slightly from the nominal value as compared with short thick segments. As stated above, this is precisely the behavior which is desired. However, this apparatus cannot detect excessively thin segments.
Adjusting this apparatus by changing the setpoints of damping and response threshold of the Schmitt trigger for a new sort of yarn is fairly time-consuming. Moreover, this apparatus is unable to grade because it can ascertain only values that make the Schmitt trigger respond, but cannot recognize acceptable yarn defects. Thus, perspective is lacking on the distribution of the accepted yarn defects and the cut yarn defects, and no conclusion may be drawn as to how close this apparatus has come to the optimum cleansing boundary.
"Selectors" are also known. These are cards receiving the actually occurring yarn defects in two dimensions, arranged by thickness deviation and yarn defect length. They are used to determine the unacceptable defects for yarn cleansing. Thus, a yarn cleansing apparatus must operate in such a way that it shall excise from the yarn all defects ascertained by the selector as being unacceptable.
European patent document B 0 153 350 discloses a grading method and the associated apparatus operating with first processors to periodically monitor an ever larger number of spindles, illustratively stated as 24, and a central processor. The grading of spindle-produced yarn always is carried out within a limited monitoring time interval, whereupon the yarn of the next spindle is graded, and so forth, until, after a comparatively long time has elapsed, the yarn from the original spindle is graded again. Hence, grading is carried out only by "sampling" over a small portion of the spindle operating time so that the checking does not detect all defects but rather is fairly statistical in nature and is useful only in steady-state operation of the individual spindles. To reduce the unreliability of such sampling, especially when starting up a spindle, the periodic checking of all other spindles is suspended during the start-up time of a spindle until it has reached its steady state. This procedure is easily carried out with a spinning machine because the spindles are started up consecutively and no more than one is starting at any given time. On the other hand, such a procedure entails further lapses in grading time for the other spindles. Because such apparatus checks the spindle-made yarn for its thickness only briefly, it is unsuitable to grade yarn for which all unacceptable defects must be detected.